Because of their unique sieving characteristics, as well as their catalytic properties, crystalline molecular sieves and molecular sieves are especially useful in applications such as hydrocarbon conversion, gas drying and separation. Although many different crystalline molecular sieves have been disclosed, there is a continuing need for new molecular sieves with desirable properties for gas separation and drying, hydrocarbon and chemical conversions, and other applications. New molecular sieves may contain novel internal pore architectures, providing enhanced selectivities in these processes.
Molecular sieves have distinct crystal structures which are demonstrated by distinct X-ray diffraction patterns. The crystal structure defines cavities and pores which are characteristic of the different species.
Molecular sieves are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework type zeolites and other crystalline microporous molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the “Atlas of Zeolite Framework Types” Sixth Revised Edition, Elsevier (2007) and the Database of Molecular sieve Structures on the International Zeolite Association's website (http://www.iza-online.org).
The structure of a molecular sieve can be either ordered or disordered. Molecular sieves having an ordered structure have periodic building units (PerBUs) that are periodically ordered in all three dimensions. Structurally disordered structures show periodic ordering in dimensions less than three (i.e., in two, one or zero dimensions). Disorder occurs when the PerBUs connect in different ways, or when two or more PerBUs intergrow within the same crystal. Crystal structures built from PerBUs are called end-member structures if periodic ordering is achieved in all three dimensions.
In disordered materials, planar stacking faults occur where the material contains ordering in two dimensions. Planar faults disrupt the channels formed by the material's pore system. Planar faults located near the surface limit diffusion pathways otherwise required in order to allow feedstock components to access the catalytically active portions of the pore system. Therefore, as the degree of faulting increases, the catalytic activity of the material typically decreases.
In the case of crystals with planar faults, interpretation of X-ray diffraction patterns requires an ability to simulate the effects of stacking disorder. DIFFaX is a computer program based on a mathematical model for calculating intensities from crystals containing planar faults. (See, M. M. J. Treacy et al., Proceedings of the Royal Chemical Society, London, A (1991), Vol. 433, pp. 499-520). DIFFaX is the simulation program selected by and available from the International Zeolite Association to simulate the XRD powder patterns for intergrown phases of molecular sieves. (See “Collection of Simulated XRD Powder Patterns for Zeolites” by M. M. J. Treacy and J. B. Higgins, 2001, Fourth Edition, published on behalf of the Structure Commission of the International Zeolite Association). It has also been used to theoretically study intergrown phases of AEI, CHA and KFI molecular sieves, as reported by K. P. Lillerud et al. in “Studies in Surface Science and Catalysis”, 1994, Vol. 84, pp. 543-550. DIFFaX is a well-known and established method to characterize disordered crystalline materials with planar faults such as intergrown molecular sieves.
The designation ZSM-48 refers to a family of disordered materials, each characterized as having a one-dimensional 10-ring tubular pore system. The pores are formed of rolled up honeycomb-like sheets of fused tetrahedral 6-ring structures, and the pore aperture contains 10 tetrahedral-atoms. Zeolites EU-2, ZSM-30 and EU-11 fall into the ZSM-48 family of zeolites.
According to Lobo and Koningsveld, the ZSM-48 family of molecular sieves consists of nine polytypes. (See, J. Am. Chem. Soc. 2002, 124, 13222-13230). These materials have very similar, but not identical, X-ray diffraction patterns. The Lobo and Koningsveld paper describes their analysis of three ZSM-48 samples provided by Dr. Alexander Kuperman of Chevron Corporation. Each of the three samples, labeled Samples A, B and C, respectively, were prepared using three different structure directing agents. Comparative Examples 2 and 3 herein below correspond to Samples A and B described in the Lobo and Koningsveld paper.
The Lobo and Koningsveld paper describes Sample A as being polytype 6, and Sample B as being a faulted polytype 6. The paper further describes the morphology of Sample A as consisting of needle-like crystals having a diameter of ˜20 nm and a length of ˜0.5 μm. The morphology of Sample B consisted of long, narrow crystals having a width of ˜0.5 μm and a length of 4-8 μm. As indicated in Comparative Examples 2 and 3 below, the scanning electron microscopy images for Samples A and B are presented herein in FIGS. 3 and 4.
Kirschhock and co-workers describe the successful synthesis of phase-pure polytype 6. (See, Chem. Mater. 2009, 21, 371-380). In their paper, Kirschhock and co-workers describe their phase-pure polytype 6 material (which they refer to as COK-8) as having a morphology consisting of long needle-like crystals (width, 15-80 nm; length, 0.5-4 μm) with a very large length/width ratio, growing along the interconnecting pore direction.
As indicated in the Kirschhock paper, molecular sieves from the ZSM-48 family of molecular sieves consist of 10-ring, 1-dimensional pore structures, wherein the channels formed by the interconnected pores extend perpendicular to the long axis of the needles. Therefore, the channel openings are located at the short ends of the needles. As the length-to-diameter ratio (also known as aspect ratio) of these needles increases, so does the diffusion pathway for the hydrocarbon feed. As the diffusion pathway increases, so does the residence time of the feed in the channels. A longer residence time results in increased undesirable hydrocracking of the feed with a concomitant reduction in selectivity.
Accordingly, there is a current need for ZSM-48 molecular sieves which exhibit lower degree of hydrocracking over known ZSM-48 molecular sieves. There is also a continuing need for ZSM-48 molecular sieves which are phase pure or substantially phase-pure, and have a low degree of disorder within the structure (a low degree of faulting).